1. Field of the Invention
The present invention relates to a polarity exchanger and an ion implanter having the same. More particularly, the present invention relates to a polarity exchanger for changing a polarity of an ion beam in an accelerator for accelerating the ion beam and an ion implanter having the same.
2. Description of the Related Art
Generally, semiconductor devices are manufactured through a fabrication process for forming electric circuits on a silicon wafer serving as a semiconductor substrate, an inspection process for inspecting electrical characteristic of the electrical circuits, and a packaging process for individually packaging the electrical circuits using an epoxy resin.
The fabrication process includes a deposition step for forming a film on the substrate, a chemical mechanical polishing step for planarizing the film on the substrate, a photolithography step for forming a photoresist pattern on the film, an etching step for etching the film to form electrical patterns using the photoresist pattern, an ion implantation step for implanting ions into predetermined portions of the substrate, a cleaning step for removing impurities from the substrate, and an inspection step for inspecting a surface of the substrate where the electrical patterns are positioned, and the like.
Among the above unit steps, the ion implantation step is executed in order to form a source/drain region of a transistor by implanting ions into the predetermined portions of the substrate. It is very important that specific ions are uniformly implanted into the portions of the substrate to form the source/drain region during the ion implantation step. Ion implantation has an advantage over conventional thermal diffusion in that an amount and depth of ions implanted into the source/drain region of the transistor may be exactly adjusted to a desired amount and depth.
An apparatus for performing an ion implantation process generally includes an ion generator, an ion extractor, a first polarity exchanger, a mass spectrometer, an accelerator, a second polarity exchanger, a focusing lens and an ion implantation chamber.
Ions generated from the ion generator are converted into an ion beam by the ion extractor. The first polarity exchanger changes a polarity of the extracted ion beam such that a negative ion beam is formed. The mass spectrometer selects specific ions from the negative ion beam and directs a negative ion beam consisting of the selected negative ions into the accelerator. A high voltage is applied to electrodes disposed in series in the accelerator to generate a Coulomb force for accelerating the negative ion beam. The second polarity exchanger is disposed adjacent to a central portion of the accelerator to change the negative ion beam into a positive ion beam. The positive ion beam changed by the second polarity exchanger is accelerated by the accelerator. The accelerated positive ion beam is focused on a semiconductor substrate through the focusing lens.
The polarity exchanger typically includes a stripping canal disposed in the accelerator, a gas supply unit for providing a stripping gas used to change the negative ion beam into the positive ion beam, and a gas circulation unit for circulating the stripping gas provided into the stripping canal.
The gas supply unit includes a gas source for storing the stripping gas, a flow control valve and a gas supply pipe. The gas supply unit further includes a regulator for maintaining a constant pressure of the stripping gas. The regulator maintains the stripping gas at a pressure of about 175 psi. The flow control valve controls a flow rate of the stripping gas to be about 0.3 to about 0.4 sccm. The stripping gas includes a nitrogen gas or an argon gas.
The stripping gas provided into the stripping canal collides with the negative ion beam accelerated by the accelerator so that the negative ion beam is converted into the positive ion beam.
Meanwhile, the gas circulation unit is disposed between the stripping canal and the gas supply pipe to circulate the stripping gas provided into the stripping canal. The gas circulation unit circulates the stripping gas so that collisions between the stripping gas and the electrodes of the accelerator may be prevented, and metallic contaminants are not generated from the electrodes of the accelerator.
The gas circulation unit includes a gas circulation pipe for circulating the stripping gas, a vacuum pump for sucking the provided stripping gas into the stripping canal, a generator for applying power to operate the vacuum pump, a motor for operating the generator and a power source for operating the motor.
When the vacuum pump of the gas circulation unit begins to deteriorate during an ion implantation process employing the ion implantation apparatus, a driving current being applied to the vacuum pump may be augmented and a flow rate of the stripping gas may also be increased. The increase in flow rate of the stripping gas may cause a reduction in a polarity exchange rate and a transmission rate. Additionally, the stripping gas may collide with the electrodes of the accelerator as a circulation efficiency of the stripping gas is reduced. As a result, the aforementioned metallic contaminants are generated from the electrodes of the accelerator. Consequently, the semiconductor substrate may be polluted by the metallic contaminants, causing a failure in a semiconductor device.